The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with therapeutic substance therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life. One type of medical device is an implantable therapeutic substance infusion device.
An implantable therapeutic substance infusion device is implanted by a clinician into a patient at a location appropriate for the therapy. Typically, a therapeutic substance infusion catheter is connected to the device outlet and implanted to infuse the therapeutic substance such as a drug or infusate at a programmed infusion rate and predetermined location to treat a condition such as pain, spasticity, cancer, and other medical conditions. Many therapeutic substance infusion devices are configured so the device can be refilled with therapeutic substance through a septum while the device is implanted. Then the time the device can be implanted may not be limited by therapeutic substance stored capacity of the device. An example of an implantable therapeutic substance infusion is shown in Medtronic, Inc. product brochure entitled “SynchroMed® Infusion System” (1995).
Electrically powered implanted therapeutic substance infusion devices consume energy delivered typically by a battery, also called a power source, and can require replacement once implanted due to depletion of the battery. Typically the most significant power-consuming component in an implantable infusion device is the therapeutic substance metering motor such as a stepper motor.
A stepper motor is an electromechanical device whose rotor rotates a discrete angular amount when an electrical drive pulse is applied to the stator windings. The amplitude and the width of the electrical drive pulse must be tailored to the electromechanical properties of the motor in order to achieve rotation, stability, and optimal energy consumption. Examples of instability include the motor rotating backwards, stepping ahead then “flipping back” to its starting position, and not stepping at all. For a stepper motor to function normally and efficiently over a wide power source voltage range, the motor drive pulse needs to be adjusted proportional to the voltage change of the power source.
If all motor drive pulse parameters are held constant while the power source voltage decreases, a decrease due to normal consumption of power source energy, excess energy above that needed by the motor is delivered at the beginning of the service life of the device. This occurs because the pulse parameters needed for the end of service life, for example, pulse width, are greater than needed at the beginning.
Thus, unless the pulse parameters are appropriately varying as the power source voltage is varying, the excess energy drawn from the power source undesirably reduces the service life of the implantable pump. This may cause an early need to replace the pump which is undesirable.
Since replacement of the implanted device requires an invasive procedure of explanting the existing device and implanting a new device, it is desirable to extend battery life to the greatest extent practicable. Some previous implantable infusion devices have reduced power consumption by varying the pulse width of the motor drive signal, but stepper motors can become unstable or stall under some circumstance when the pulse width is varied from the optimal pulse width that the motor is typically designed to use. An example of a motor drive signal with a varying pulse width is shown in Japanese Patent 11,042,286 “Intracorporealy Embedded Type Liquid Medicine Supplying Apparatus” by Yamazaki (Feb. 16, 1999).
For the foregoing reasons, there is a need for an implantable therapeutic substance infusion device with optimized pump motor drive to increase the infusion device's effective service life. This increased service life reduces the overall cost of the medical therapy and the inconvenience to the patient and clinician for future device replacement surgeries.